Final answer:
The changes in the sound of a siren are explained by the Doppler effect, which is the variation in observed frequency of sound that occurs due to the relative motion between the source and the observer, resulting in a change in pitch as the source approaches and moves away.
Step-by-step explanation:
When asking about the changes in sound from a siren, we're referring to the Doppler effect. The Doppler effect is a change in the observed frequency of a sound due to the relative motion between the source of the sound and the observer.
It occurs because sound waves bunch up (increase in frequency) as the source approaches you and spread out (decrease in frequency) when it moves away from you. This shift in frequency is perceived by our ears as a change in pitch: higher as the source approaches and lower as it recedes.
When a vehicle equipped with a siren, like an ambulance or a police car, passes by you, the pitch of the siren's sound appears to shift. This shift happens even though the siren emits a constant frequency. The closer the vehicle comes to you and the faster it moves, the more drastic the pitch change.
For instance, as an ambulance approaches, you will perceive the siren’s pitch as becoming higher, and as it passes and moves away, the pitch will drop, despite the actual frequency of the sound remaining constant.
Therefore, the correct answer to the question regarding the explanation for the way you are hearing the sound from the siren is a) Doppler effect.